Cooling and refrigeration requirements of industrial installations, as well as the central air conditioning of commercial buildings and industrial plants, requires large amounts of electrical energy to operate the refrigeration plants needed for these purposes. This places a high demand on electric utilities during on-peak periods, which usually are from about 9 A.M. to 10 P.M. Monday through Friday. Utilities must provide enough generating capacity to meet this demand. This requires a very high capital investment for plants and equipment which are fully utilized only in hot weather in daylight hours. Evenings and weekends are off-peak demand periods and much less of the total generating capacity is used then. In addition, less generating capacity is used on cool days in the spring and fall periods of the year in the United States.
To encourage a better or more uniform demand for electric power, many utilities charge a reduced rate for electricity used during off-peak periods. Business and industry have accordingly been looking for ways to shift or transfer as much as possible of their electrical consumption to off-peak periods to take advantage of the reduced rates and also to minimize future electric rate increases by making additional electric generating plants unnecessary, or at least delaying generating plant expansion.
It has been recognized for some time that a substantial potential savings could be realized if much of the refrigeration or air conditioning load could be moved from on-peak to off-peak periods. To do this, it has been proposed to operate refrigeration plants during off-peak periods to produce cold or chilled water or ice for storage. During on-peak periods the cold or chilled water or ice would then be used to provide cooling. Because ice provides greater cooling capacity per unit volume than chilled water (a ratio of about 7:1) much commercial interest has been directed toward providing so-called ice building equipment for this purpose.
At this time it appears that the type of ice builder of greatest interest, and one which has been put into use in a number of installations, constitutes a tank, for holding water, through which a large number of small pipes run in one of several different patterns or arrangements. A liquid refrigerant is fed through the small pipes. As the refrigerant absorbs heat from the water, a layer of ice about 1 to 3 inches thick forms on each pipe. Ice is produced in this manner during off-peak periods.
When it becomes desirable to utilize the cooling potential stored in the ice for air conditioning or other purposes, a stream of water is fed through the tank to cool the water by exchange of heat to the ice. The cooled water is withdrawn from the tank and fed to a heat exchanger to cool or air condition a building or for other cooling purposes. The resulting warm water is then returned to the tank to be cooled again by contact with the ice. This system can continue to provide cooling until all the ice is melted.
Ice builders of the described type are costly to fabricate and operate. The pipes are not readily repaired or serviced. In addition, as the ice layer on the pipes increases in thickness, heat exchange between the water and refrigerant decreases because of the insulating effect which the ice provides. Furthermore, a very large heat exchange surface must be provided by the pipes to obtain the cooling needed to produce the desired quantity of ice.
From the above it is clear that a need exists for alternative apparatus and methods for cooling which can operate with nearly constant efficiency during off-peak load, or even during on-peak load if desired.